Multi-frequency transmission/reception apparatus

ABSTRACT

The invention provides a multi-frequency transducer capable of obtaining desired directivity regardless of frequencies. Parallelepiped-shaped transducer elements ( 1 ) are used as transducer elements and a plurality of such transducer elements ( 1 ) are arranged in an array. The transducer elements ( 1 ) are caused to resonate using a resonant frequency in fundamental vibration mode and a resonant frequency in harmonic vibration mode determined by the dimensions (A, B) of a short edge ( 1   a ) and a long edge ( 1   b ) perpendicular to a longitudinal edge ( 1   c ) of each transducer element ( 1 ) to transmit and receive at multiple frequencies. The angle of directivity (θ) of the transducer elements is controlled by the dimension (C) of the longitudinal edge which is set to a value which does not affect resonance in either the fundamental vibration mode or the harmonic vibration mode.

TECHNICAL FIELD

[0001] The present invention relates to a multi-frequency transducerused in a fish-finding echo-sounder or a sonar and, in particular, to amulti-frequency transducer including a plurality ofparallelepiped-shaped transducer elements arranged in an array.

BACKGROUND ART

[0002] A multi-frequency transducer is a type of transducer which isoperated on a frequency selected in accordance with targets of search.This type of transducer is used in underwater sounding apparatus, suchecho-sounders and sonars. A typical example of the multi-frequencytransducer is a two-frequency transducer which is designed to transmitand receive ultrasonic waves at either of two frequencies. Although thetwo-frequency transducer of this kind may be provided with twotransducer elements designed to operate independently of each other ontwo frequencies, this approach results in an increase in the overallphysical size and cost of the transducer. In this situation, atransducer capable of transmitting and receiving ultrasonic waves at twofrequencies with a single transducer element is currently available forpractical use.

[0003] A transducer 100 shown in FIG. 11 has conventionally been used asthe aforementioned kind of transducer. The transducer 100 is constructedof a piezoelectric ceramic material (PZT), for example, which is shapedinto a circular disc having a diameter L1 and a thickness L2. Thistransducer 100 has a natural resonant frequency f1 determined by thevalue L1 as well as a natural resonant frequency f2 determined by thevalue L2, wherein there is a relationship f1<f2 between the two resonantfrequencies because L1>L2. Therefore, if a transmit signal of whichfrequency is equal to one of the two resonant frequencies f1, f2 isintroduced into the transducer 100, it resonates at the appliedfrequency and radiates ultrasonic waves at the frequency f1 or f2whichever applied. When receiving echo signals reflected back fromunderwater objects, the transducer 100 resonates again at the frequencyf1 or f2, whichever applied, and produces a receive signal at thatfrequency. It is possible to transmit and receive ultrasonic waves atthe two frequencies with this single-element transducer 100.

[0004] However, the transducer 100 has only two distinct dimensions, L1and L2, since it is shaped into a circular disc. The values L1 and L2are uniquely determined when the resonant frequencies are determined. Onthe other hand, the aforementioned disc-shaped transducer 100 normallyradiates ultrasonic waves from its circular surfaces, so that the angleof directivity of the transducer 100 is determined by its dimension L1.Since the values L1 and L2 are determined by the resonant frequencies,it is impossible to freely select the angle of directivity as a functionof the value L1. Thus, the dimensions and the angle of directivity ofthe conventional transducer are not mutually independent. Since theangle of directivity is automatically determined when the resonantfrequency is determined, it has been impossible to obtain a desiredangle of directivity.

[0005] This invention has been made in light of the aforementionedproblems. Accordingly, it is an object of the invention to provide amulti-frequency transducer of which dimensions and angle of directivityare made mutually independent so that a desired angle of directivity canbe obtained regardless of frequencies.

DISCLOSURE OF THE INVENTION

[0006] To solve the aforementioned problems, parallelepiped-shapedtransducer elements are used as transducer elements and a plurality ofsuch transducer elements are arranged in an array in the presentinvention. The transducer elements are caused to resonate at a resonantfrequency in fundamental vibration mode and at a resonant frequency inharmonic vibration mode determined by the dimensions of two edges ofeach transducer element perpendicular to a longitudinal edge of eachtransducer element to transmit and receive at multiple frequencies. Thelongitudinal edge is set to a dimension which does not affect resonancein either the fundamental vibration mode or the harmonic vibration mode,and the angle of directivity of the transducer elements is controlled bythe dimension of the longitudinal edge.

[0007] Since there are three dimensions when such parallelepiped-shapedtransducer elements of the invention are used, it is possible to freelydetermine one dimension even when two dimensions have been determinedaccording to resonant frequencies. This makes it possible to select theangle of directivity by varying that dimension regardless of theresonant frequencies. While harmonic vibration modes include thirdharmonic vibration mode, fifth harmonic vibration mode, and so on, theexpressions third harmonic vibration mode, fifth harmonic vibrationmode, etc. as used in this Description are not limited in the strictsense to the third harmonic vibration mode, fifth harmonic vibrationmode, etc. defined in physics but imply broader concepts includingapproximate third harmonic vibration mode, approximate fifth harmonicvibration mode, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008]FIG. 1 is a perspective view of a parallelepiped-shaped transducerelement used in a multi-frequency transducer of the present invention;

[0009]FIG. 2 is a plan view of the multi-frequency transducer employingparallelepiped-shaped transducer elements;

[0010]FIG. 3 is a diagram for explaining a grating lobe;

[0011]FIG. 4 shows an example of resonance characteristics of theparallelepiped-shaped transducer element;

[0012]FIG. 5 is a graph showing the relationship between a dimensionratio and array constants;

[0013]FIG. 6 is a graph showing frequency characteristics oftransmitting voltage response;

[0014]FIG. 7 is a graph showing the relationship between the dimensionratio and the transmitting voltage response;

[0015]FIG. 8 is a diagram showing how the transducer element resonatesin third harmonic vibration mode;

[0016]FIG. 9 is a plan view showing another embodiment of the presentinvention;

[0017]FIG. 10 is a plan view showing another embodiment of the presentinvention; and

[0018]FIG. 11 is a perspective view showing a conventional transducer.

BEST MODES FOR CARRYING OUT THE INVENTION

[0019]FIG. 1 is a perspective diagram showing an example of aparallelepiped-shaped transducer element used in a multi-frequencytransducer of the invention. Constructed of a piezoelectric ceramicmaterial (PZT), for example, the parallelepiped-shaped transducerelement 1 is formed into a rectangular parallelepiped shape the lengthsof which three edges are A, B and C, wherein “A” is the dimension ofshort edges 1 a, “B” is the dimension of long edges 1 b and “C” is thedimension of longitudinal edges 1 c. Designated by 1 d is a radiatingsurface for radiating ultrasonic waves defined by the edges 1 a and 1 c.The ultrasonic waves are radiated from this radiating surface 1 d in adirection shown by an arrow in the Figure. Indicated by θ is the angleof directivity of the transmitted waves. Designated by 1 e is a pair ofelectrode surfaces defined by the edges 1 b and 1 c that are located onopposite sides of the short edges 1 a. (FIG. 1 shows one of theelectrode surfaces 1 e only.) Lead wires (which will be described later)for applying a voltage to the parallelepiped-shaped transducer element 1in each transmit cycle and for taking out a voltage from theparallelepiped-shaped transducer element 1 in each receive cycle areconnected to these electrode surfaces 1 e. The direction of polarizationof the parallelepiped-shaped transducer element 1 is parallel to theshort edges 1 a as illustrated.

[0020]FIG. 2 is a plan view of the multi-frequency transducer employingthe aforementioned parallelepiped-shaped transducer element 1. Themulti-frequency transducer 10 is constructed of a transducer array inwhich a plurality of parallelepiped-shaped transducer elements 1 arearranged side by side. The individual parallelepiped-shaped transducerelements 1 are arranged in such a manner that the direction of theirshort edges 1 a coincides with an array direction Z in which thetransducer elements are arranged. The letter “d” in the Figurerepresents the interval (center-to-center distance) between theparallelepiped-shaped transducer elements 1. Designated by 2 a and 2 bare the aforementioned lead wires which are electrically connected tothe electrode surfaces 1 e (refer to FIG. 1) of theparallelepiped-shaped transducer element 1. While the lead wires 2 a, 2b are shown for the parallelepiped-shaped transducer element 1 at theextreme left end only in the Figure, the lead wires 2 a, 2 b are alsoconnected to the other parallelepiped-shaped transducer elements 1 inthe same fashion.

[0021] In the array-type transducer shown in FIG. 2, the array elementinterval d between the successive parallelepiped-shaped transducerelements 1 is an important factor for determining directionalcharacteristics. It is therefore necessary to properly select the arrayelement interval d. Specifically, when the parallelepiped-shapedtransducer elements 1 are operated with signals of the same phase,ultrasonic waves 3 incident from an oblique direction (angle φ) to theindividual parallelepiped-shaped transducer elements 1 arranged at thearray element interval d arrive with a phase difference corresponding toa distance x=d·cos φ between the adjacent transducer elements 1. Thus,when the array element interval d is large, there exists a value of φ atwhich the value x becomes an integer multiple of the wavelength λ of theultrasonic waves. The ultrasonic waves 3 incident at this angle φ areall matched in phase when they arrive at the individual transducerelements 1 and, therefore, the transducer produces a high receivedvoltage level at that incident angle φ. Since there exists the angle φof the oblique direction other than the frontal direction (φ=90°) atwhich the incident ultrasonic waves are matched in phase as seen above,there occurs a maximum point of sensitivity (grating lobe; hereinafterreferred to as GL) which leads to a deterioration in S/N ratio withrespect to the directional characteristics.

[0022] To avoid the aforementioned problem, it is preferable to make thearray element interval d smaller than the wavelength λ of the ultrasonicwaves. Specifically, because the maximum value of cost is 1, λ becomessmaller than the value x and x does not become an integer multiple of λif d is smaller than λ. If the array element interval of theparallelepiped-shaped transducer elements 1 is d (mm), the usedfrequency is f (kHz), and the sound velocity through a propagationmedium is v (m/s), the wavelength λ (mm) is expressed by λ=v/f. Thus, toavoid the occurrence of GL for ultrasonic waves incident from an obliquedirection, the parallelepiped-shaped transducer elements 1 should bearranged such that the array element interval d satisfies the followingrelationship:

d<v/f  (1)

[0023] Assuming that no sound insulator is provided between theindividual transducer elements, the array element interval d can beregarded as being equal to the dimension A of the short edge 1 a, sothat inequality (1) above can be rewritten as follows:

A<v/f

[0024] Transforming this inequality, we obtain

A·f<v  (2)

[0025] Inequality (2) above gives a condition for not causing GL tooccur. If the propagation medium is water, the underwater sound velocityis v=1500. Substituting this into inequality (2), we obtain

A·f<1500  (3)

[0026] It is possible to avoid the occurrence of GL if the transducerarray as illustrated in FIG. 2 is configured with the dimension A of theshort edges 1 a of the parallelepiped-shaped transducer elements 1 soselected as to satisfy inequality (3) above. For the convenience oflater explanation, A·f of inequalities (2) and (3) is referred to as anarray constant which is hereby defined as follows:

N=A·f

[0027] Since the parallelepiped-shaped transducer elements 1 arearranged such that the direction of their short edges 1 a coincides withthe array direction Z in FIG. 2, it is possible to limit the overallarray length in the array direction Z in which multiple transducerelements are arranged. Also, as can be recognized from FIG. 1, eachelectrode surface 1 e is a surface having the largest area defined bythe longitudinal edges 1 c and the long edges 1 b, so that the impedanceof the transducer elements can be reduced. Since the electrode surfaces1 e are surfaces perpendicular to the radiating surface 1 d and orientedsideways, the lead wires 2 a, 2 b can be easily connected. Furthermore,the radiating surface 1 d for radiating the ultrasonic waves extends inthe longitudinal direction of each transducer element. As will bedescribed later, the dimension C in the longitudinal direction does notinfluence the resonant frequency of the transducer element, so that itis possible to control its directivity independently. Based on theforegoing, it is most effective and desirable to arrange theparallelepiped-shaped transducer elements 1 of FIG. 1 in a mannerillustrated in FIG. 2.

[0028] Next, resonance characteristics of the parallelepiped-shapedtransducer elements 1 that constitute a characteristic feature of theinvention are described.

[0029] The parallelepiped-shaped transducer element 1 shown in FIG. 1has three dimensions A, B and C and, accordingly, has natural resonantfrequencies corresponding to the individual lengths. In this invention,multiple frequencies are obtained by use of the individual resonantfrequencies in fundamental vibration modes and harmonic vibration modesdetermined by the dimension A of the short edge and the dimension B ofthe long edge, and the angle of directivity θ is controlled by thelongitudinal dimension C. A specific embodiment given below is anexample of a two-frequency system.

[0030] Referring to FIG. 1, the dimension A of the short edge and thedimension B of the long edge of the parallelepiped-shaped transducerelement 1 are set to have a dimension ratio, such as A/B=0.1, which willnot produce later-described combination vibration. Also, thelongitudinal dimension C is set to a value which will not influenceresonance in either the fundamental vibration mode or harmonic vibrationmode, such as to satisfy C>3B.

[0031]FIG. 4 shows an example of resonance characteristics of such theparallelepiped-shaped transducer element 1, in which the horizontal axisrepresents the frequency of a signal applied to theparallelepiped-shaped transducer element 1 and the vertical axisrepresents the absolute value of the impedance of the transducerelement. f-a, f-b, . . . , f-f represent vibration modes and fr_a, fr_b,. . . , fr_f represent resonant frequencies in the individual vibrationmodes. Among these vibration modes, f-a and f-b are fundamentalvibration modes determined by the dimension A of the short edge and thedimension B of the long edge, respectively, and f-c, f-d, f-e and f-fare harmonic vibration modes. It is to be noted that although resonancein the fundamental vibration mode determined by the longitudinaldimension C of the transducer element appears at a frequency lower thanfr_b, this resonance is omitted in FIG. 4.

[0032] The parallelepiped-shaped transducer element 1 has the resonantfrequencies not only in the fundamental vibration modes but also in theharmonic vibration modes as seen above. Therefore, it is possible torealize a two-frequency transducer by using the resonant frequency fr_bin the fundamental vibration mode f-b and the resonant frequency fr_c inthe third harmonic vibration mode f-c, for example. Specifically, if atransmit signal of the frequency fr_b and a transmit signal of thefrequency fr_c are alternately applied to each parallelepiped-shapedtransducer element 1 of FIG. 2 through the lead wires 2 a, 2 b, thetransducer elements resonate at the frequency fr_b or fr_c and radiateultrasonic waves underwater at the relevant frequency. When receivingecho signals reflected back from underwater objects, the transducerelements resonate again at the frequency fr_b or fr_c and produces areceive signal at the relevant frequency.

[0033] While there exists the fundamental vibration mode f-a determinedby the dimension A besides the fundamental vibration mode f-b determinedby the dimension B, the resonant frequency fr_a in the fundamentalvibration mode f-a can not be used because it does not satisfy theaforementioned condition concerning the array constant (inequality (3)).Also, although there exist the fifth harmonic vibration mode f-d, theseventh harmonic vibration mode f-e, and so on besides the thirdharmonic vibration mode f-c, a usable range of the fifth harmonicvibration mode f-d is limited and seventh and higher harmonic vibrationmodes can not be used due to restrictions imposed by the conditionconcerning the array constant. This is further described in thefollowing.

[0034]FIG. 5 is a graph showing the relationship between the value ofA/B and the array constant, in which Na, Nb, . . . , Nf represent arrayconstants corresponding to the individual vibration modes f-a, f-b, . .. , f-f. Since the array constant is defined by the equation N=A·f aspreviously mentioned, the individual array constants Na, Nb, . . . , Nfare given by calculating products of the dimension A of the short edgeof the transducer element and its resonant frequencies fr_a, fr_b, . . ., fr_f.

[0035] As will be recognized from FIG. 5, the array constant Nb of thefundamental vibration mode f-b does not exceed 1500 even if A/B varieswithin a range of 0.1 to 1.0. This means that the array constant Nbsatisfies the condition of inequality (3) for not causing GL to occur.Similarly, the array constant Nc of the third harmonic vibration modef-c does not exceed 1500 if A/B falls within the range of 0.1 to 1.0, sothat the array constant Nc also satisfies the condition of inequality(3). Although the array constant Nd of the fifth harmonic vibration modefd does not exceed 1500 if A/B falls within a range of 0.1 to 0.4, thearray constant Nd exceeds 1500 and does not satisfy the condition ofinequality (3) if A/B becomes equal to 0.4 or above. Further, the arrayconstant Na of the fundamental vibration mode f-a and the arrayconstants Ne, Nf of the seventh and higher harmonic vibration mode f-e,f-f exceed 1500 regardless of the value of A/B, so that the arrayconstants Na, Ne, Nf do not satisfy the condition of inequality (3).

[0036] It is understood from the foregoing that the fundamentalvibration mode f-b, the third harmonic vibration mode f-c, and part ofthe fifth harmonic vibration mode f-d satisfy the condition ofinequality (3). Accordingly, a two-frequency transducer which does notproduce GL is realized by using two frequencies, that is, the resonantfrequency fr_b in the fundamental vibration mode f-b and the resonantfrequency fr_c in the third harmonic vibration mode f-c in theaforementioned embodiment.

[0037] On the other hand, the angle of directivity θ can be regulated byvarying the longitudinal dimension C of the transducer elements. Sincethe dimension C is set to a value which does not influence the resonantfrequencies fr_b, fr_c in the fundamental vibration mode f-b and thethird harmonic vibration mode f-c, it is possible to set the dimension Cindependently of the resonant frequencies. Specifically, although thedimensions A and B are restricted by the two resonant frequencies fr_b,fr_c, the dimension C is not restricted by the resonant frequenciesfr_b, fr_c, so that the length can be freely determined. Consequently,the angle of directivity θ determined by the dimension C can be setindependently of the resonant frequencies. While the angle ofdirectivity θ is an angle of directivity along the longitudinaldirection of each parallelepiped-shaped transducer element 1, an angleof directivity along the array direction Z (refer to FIG. 2) of theparallelepiped-shaped transducer elements 1 can be regulated by thenumber of the parallelepiped-shaped transducer elements 1 to bearranged, so that this angle of directivity is not restricted by theresonant frequencies either. Since the dimensions A, B and the dimensionC can be determined mutually independently, it is possible to realize atransducer which permits a combination of desired resonant frequenciesand desired angles of directivity.

[0038] Shown in FIG. 6 is an example of transmitting voltage response inthe individual vibration modes representing frequency characteristics ofthe transmitting voltage response on a central acoustic axis of thetransducer. Whereas FIG. 6 shows the characteristics obtained when thevalue of A/B is fixed, FIG. 7 shows how the transmitting voltageresponse varies when the value of A/B is varied. Shown in FIG. 7 is thetransmitting voltage response in the third harmonic vibration mode f-c.As can be seen from FIG. 7, the transmitting voltage response remains at150 dB or above when the value of A/B is within a range of 0.1 to 0.5,whereas the transmitting voltage response becomes lower than 150 dB andsensitivity drops when the value of A/B exceeds 0.5. Practically, thecharacteristics are good enough for a two-frequency transducer if thetransmitting voltage response is 150 dB or above and, therefore, it ispreferable to set the value of A/B to 0.5 or below when using the thirdharmonic vibration mode f-c. It should however be noted that the value0.5 is just one example. Generally, the value of A/B is set to a valueequal to or smaller than a specific value which is predetermined basedon the transmitting voltage response.

[0039]FIG. 8 is a diagram showing how the transducer element resonatesin the third harmonic vibration mode f-c, in which (a) and (b) show howthe transducer element resonates when A/B is 0.3 and 0.6, respectively.While vibration in clear third harmonic vibration mode is produced in(a), no vibration in harmonic vibration mode can be seen in (b). Thevibration of (b) seems to be produced in combination vibration mode inwhich some vibration modes are combined. Vibrating efficiency is poor inthis kind of combination vibration mode. A drop in the transmittingvoltage response that occurs when the value of A/B exceeds 0.5 as shownin FIG. 7 is supposed to be caused by the occurrence of the combinationvibration mode. Therefore, it is possible to prevent the occurrence ofthe aforementioned combination vibration mode by setting the dimensionratio A/B of the short edge to the long edge of eachparallelepiped-shaped transducer element 1 to 0.5 or less.

[0040] Shown in FIG. 9 is another embodiment of the present invention,in which an arrangement is made to avoid the occurrence of GL andsuppress side lobes. In this embodiment, the longitudinal dimension ofindividual parallelepiped-shaped transducer elements 1 is successivelyvaried along the array direction Z to assign them specific weights inthe form of varying shapes. It is possible to control directivity of thetransducer elements 1 in the array direction Z and suppress side lobesby assigning weights in this manner. Since the longitudinal dimension ofthe transducer elements 1 does not affect their resonant frequencies asstated earlier, it is possible to control the directivity and suppressside lobes regardless of the frequency by arranging the transducerelements 1 having varying dimensions as illustrated.

[0041] Weights can be assigned either by the aforementioned arrangementof varying shapes or by an electrical weighting method. FIG. 10 shows anembodiment employing this electrical weighting method, in whichparallelepiped-shaped transducer elements 1 are divided in a directionperpendicular to the array direction Z so that there are provided aplurality of transducer arrays 11, 12, 13 each including an arrangementof multiple transducer elements. In this embodiment, the magnitude ofvoltage applied to the individual transducer arrays 11, 12, 13 is variedto weight the applied voltage. It is already known, as described inJapanese Unexamined Patent Publication No. H5-60858 and JapaneseExamined Patent Publication No. H3-23874, for example, that thedirectivity can be improved by weighting the applied voltage by varyingsignal voltages applied to the transducer elements. Known methods can beused as means for weighting, such as a method of using the ratio of thenumbers of windings of a transformer and a method of controlling theimpedance of transducer elements. Also, functions such as Chebyshev'sfunction and Gaussian function can be used as a weighting function. Itis also possible to control the directivity along the array direction Zby weighting signal voltages with varying magnitudes that are applied toeach longitudinally arranged set of transducer elements a, b, . . . , jshown in FIG. 10.

[0042] It is also possible in the construction of FIG. 2 to control thedirectivity along the array direction by weighting signal voltages withvarying magnitudes that are applied to the individualparallelepiped-shaped transducer elements 1.

[0043] The present invention is not limited to the aforementionedembodiments alone but is applicable in various other forms. For example,while the examples of the two-frequency transducers using thefundamental vibration mode f-b and the third harmonic vibration mode f-chave been discussed in the aforementioned embodiments, it is alsopossible to realize a three-frequency transducer by using a resonantfrequency determined by the longitudinal dimension C of the transducerelements. Furthermore, it is possible to utilize the fifth harmonicvibration mode f-d as a harmonic vibration mode within a specific rangein addition to the third harmonic vibration mode f-c. This makes itpossible to realize a four-frequency transducer.

[0044] Also, while the parallelepiped-shaped transducer elements 1 areconstructed of the piezoelectric ceramic material (PZT) in the foregoingembodiments, any piezoelectric material, such as barium titanate, may beused as a material of the transducer elements. Since different materialshave different sound velocities, it is needless to say that the value ofA/B varies depending on the material used.

[0045] According to the present invention, it is possible to set thedimensions of parallelepiped-shaped transducer elements which determinetheir resonant frequencies and the dimension which determines theirdirectivity independently of one another. This makes it possible toprovide a multi-frequency transducer capable of realizing desireddirectional characteristics regardless of the frequency.

INDUSTRIAL APPLICABILITY

[0046] The present invention is applicable to a multi-frequencytransducer used in a fish-finding echo-sounder or a sonar, for instance.

1. A multi-frequency transducer comprising a plurality ofparallelepiped-shaped transducer elements arranged in an array, saidmulti-frequency transducer being characterized in that saidparallelepiped-shaped transducer elements are constructed such that theyresonate at a resonant frequency in fundamental vibration mode and at aresonant frequency in harmonic vibration mode determined by thedimensions of two edges of each transducer element perpendicular to alongitudinal edge of each transducer element to transmit and receiveultrasonic waves at multiple frequencies, said longitudinal edge is setto a dimension which does not affect resonance in either the fundamentalvibration mode or the harmonic vibration mode, and saidparallelepiped-shaped transducer elements have an angle of directivitydetermined by the dimension of said longitudinal edge.
 2. Themulti-frequency transducer according to claim 1 which is characterizedin that the two edges of each transducer element perpendicular to thelongitudinal edge are short and long edges and saidparallelepiped-shaped transducer elements resonate at the resonantfrequency in the fundamental vibration mode which is determined by thedimension of the long edge and at the resonant frequency in the harmonicvibration mode which is determined by the dimension of the short edge,and said short and long edges are set to a dimension ratio which doesnot produce combination vibration.
 3. The multi-frequency transduceraccording to claim 1 or 2 which is characterized in that it utilizes aresonant frequency in third harmonic vibration mode or fifth harmonicvibration mode as the resonant frequency in the harmonic vibration mode.4. The multi-frequency transducer according to any of claims 1 to 3which is characterized in that it utilizes a resonant frequency infundamental vibration mode determined by the dimension of thelongitudinal edge in addition to the resonant frequency in thefundamental vibration mode determined by the dimensions of said twoedges.
 5. The multi-frequency transducer according to claim 2 or 4 whichis characterized in that the parallelepiped-shaped transducer elementsare arranged in such a manner that the direction of their short edgescoincides with the direction of the array, surfaces defined by thelongitudinal and short edges constitute radiating surfaces for radiatingultrasonic waves, and surfaces defined by the longitudinal and longedges constitute electrode surfaces.
 6. The multi-frequency transduceraccording to any of claims 1 to 5 which is characterized in that theparallelepiped-shaped transducer elements are arranged to satisfy acondition d<v/f where d (mm) is the interval between the transducerelements, f (kHz) is a used frequency, and v (m/s) is the sound velocitythrough a propagation medium.
 7. The multi-frequency transduceraccording to any of claims 2 to 6 which is characterized in that theratio A/B of the dimension A of the short edge to the dimension B of thelong edge of each transducer element is set to a value equal to orsmaller than a specific value which is predetermined based ontransmitting voltage response.
 8. The multi-frequency transduceraccording to any of claims 1 to 7 which is characterized in that theparallelepiped-shaped transducer elements are weighted along thedirection of the array in the form of varying shapes or electrically.